What are the rules for extending a Thai visa and how does the 30 days calculation work after border crossing?

Our understanding from reading here was that we didn't need to wait until the last couple days to go extend, border bounce, etc., as the new 30 days (usa) would be figured from the previous end date and not the day of crossing. We went up to Chiang Rai and crossed at Mae Sai. They only stamped us to Dec 15th, not the 23rd as it should have been. 😢